The invention is directed to printed circuit boards (PCB) or the like, to electronic devices for interconnecting PCBs, to devices for mounting PCBs onto a substrate, and to processes for solder attaching electronic components to a substrate. The invention is also particularly concerned with attaching a daughter card to a motherboard, especially to permanently attach a daughter card perpendicular to a motherboard.
Nowadays, there are several ways to attach a daughter card perpendicular to a motherboard. These include the following.
1. Removable Edge Card Connectorxe2x80x94There are available several removable edge card connector designs where the daughter card such as a memory card, arranged perpendicular to the motherboard, is inserted into a group of pins or contacts that are pre-loaded with a determined amount of force. These contacts are held in place by a molded insulator and hard soldered to the motherboard. The result is a non-soldered, removable interconnect between the memory card or edge card connector and the PCB.
2. Submerged Solderingxe2x80x94Another method of lead attachment comprises inserting the memory card or other PCB with surface mount technology (SMT) pads between two rows of contacts that are held together in a carrier, which is sometimes also referred to as a header. This assembly is fluxed and then submerged into a solder bath up to the SMT pads on the memory card or PCB. This approach results in a permanent soldered interconnect, which, however, requires that the PCB""s or memory cards be singulated and put through a secondary process, off-line, which is time-consuming and expensive.
3. Hand Solderingxe2x80x94This attachment method requires a single point solder reflow for each individual contact. An operator would touch a soldering iron and solder wire to make each solder joint. This method is slow and very labor intensive and also requires secondary processing off-line.
4. Screen Print/ Solder Bumpxe2x80x94In this scheme, during placement of other components on the PCB, solder paste is placed on the SMT pads on the xe2x80x9cAxe2x80x9d side of a printed circuit board (the terms xe2x80x9cAxe2x80x9d side and xe2x80x9cBxe2x80x9d side are meant to designate the two major surfaces of a PCB, with the A side representing the first surface processed and the B side the opposite surface) components are placed onto the paste and then the assembly is reflowed. xe2x80x9cReflowingxe2x80x9d is a term of art typically meaning that an assembly is fluxed and then heated to a temperature above the melting point of any solder present causing the solder to penetrate adjacent crevices between solderable electrically-conductive surfaces, typically of copper or a copper alloy, such that upon cooling the solder freezes forming a permanent electrical connection between the electrically-conductive surfaces. The PCB board is then flipped,wet screen paste is applied on the B side, components are placed on the B side, and then a connector for the memory card is placed over the bumps on the A side SMT pads and their leads are placed into the xe2x80x9csoftxe2x80x9d paste on the SMT pads on the B side, followed by both sides being reflowed. This process does not produce a desirable solder attachment because the leads of the connector are spread when placed over the bumps to the memory card leaving a space or gap between the pin contact region and the SMT pad after reflow.
A principal object of the invention ,is to solder-attach leads to the edge contacts of a PCB, such as a memory card, for attachment to another PCB, such as a motherboard.
Another object of the invention is to solder-attach leads to the edge contacts of a PCB, such as a memory card, for perpendicular attachment to a motherboard without the need to add solder during this last process.
A further object of the invention is an in-line process for permanently soldering the adjoining contact surfaces of electrical components by reflowing solder without separately adding directly to the joint additional solder during the reflow step to fabricate a robust solder joint between the components.
These and other objects are achieved in accordance with one feature of the invention by a solder transfer method which uses an electrically-conductive, e.g., metal, surface, stamping or pin or pad as a vehicle to transfer molten solder from a remote solder reserve to a solder joint to be made.
In a preferred embodiment of the invention, a solder deposit previously-formed on a surface mount pad during a first reflow will transfer up to the adjoining contact surfaces by a wicking process to provide the additional solder needed to fabricate a robust solderjoint. When, for example, one of the contact surfaces is a contact or pin on a first PCB or carrier, and the other contact surface is a SMT pad on a second PCB serving, for example, as a memory card, the process of the invention makes for an easy SMT processed attachment of the memory card to the first PCB without the need to add solder during a second reflow step (only requiring fluxing before reflow) to make a robust solderjoint between the contact surfaces.
In accordance with another preferred embodiment of the invention, a PCB, for example, with traces and plated through-holes, serving as a carrier or header, can be employed as the first PCB to provide a change-spacing method for the center to center and row to row spacing from the top to the bottom of the carrier. In other words, the top spacing can be chosen to match the contact spacing of the second PCB, and the bottom spacing can be chosen to match the (different) contact spacing of a third PCB on which the second PCB is to be mounted. Again, no additive solder processing is required. Alternatively, SMT interconnects rather than plated through-holes can be employed.
In accordance with another feature of the invention, a carrier with dual rows of pins forming contact surfaces for receiving the edge connector of a card provides the additional solder as a reflowed deposit on side surfaces of the pins.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described the preferred embodiments of the invention, like reference numerals or letters signifying the same or similar components.